The invention concerns a method of manufacturing glass-lined metal tubes in which the inserted glass tube is formed into a continuous fused coating over the entire inner surface of the metal tube and over the surfaces of flanged joints.
Glass-lined metal tubes are widely used in many fields of the chemical engineering industry for the transfer of corrosive fluids and for heat transfer applications owing to their combined characteristics of high resistance to corrosion inherent in the glass and the mechanical strength offered by the metal tube. Further advantages are seen in the lack of scaling as a result of the smoothness of the glass lining and the strength of the tubes under conditions of heat.
Glass-lined metal tubes at present in general use fall into two main categories as far as the method adopted for the glass lining is concerned. The first method is one whereby the inner surface of the metal tube and the faces of flange joints are coated with a glazing agent which is then dried. The glazing agent is then calcined at high temperature to cause it to adhere to the surface of the metal. The process steps are repeated thus providing a multi-layer coating of agglutinated glass on the metal base. In the second method, a glass tube whose outer diameter is slightly smaller than the inner diameter of the metal tube is inserted into the latter, softened by heating and allowed to expand by normal air pressure so that it will become attached to the inner wall of the metal tube. Modifications of this method are described in U.S. Pat. Nos. 2,986,847 and 3,235,290.
The first of the above methods is a similar process to that which has been adopted for providing glass linings for containment vessels and it may also be used not only for straight tube sections but for valves, joints of various configurations, and components for pipe systems in which the diameters of the pipes and flanges are dissimilar. Because the shape of the flange faces is the same as the nozzle components of the glass lined containment vessels and because the glass coating is continuous up to the faces of the flange junctions, these types of glass-lined components are extensively used as accessories in such containment facilities.
However, when this method is adopted for the manufacture of small diameter and long lengths of glass-lined metal tube, it is difficult to ensure total covering of the interior of straight pipe sections and there is also some difficulty in inspection and rectification. The process is also expensive in that several repetitions of the glazing and calcination steps are necessary to ensure that all pinholes have been eliminated.
With the second method, on the other hand, the glass-lining process can be completed by a single heating stage in which the inserted glass tube is expanded and attached against the inner wall of the metal tube. There is no risk of the occurrence of pinholes in the glass lining in straight sections and there is also virtually no danger of damage occurring to the glass lining in straight sections when the tube is in use. The disadvantage, however, lies in the fact that there is a difficulty in extending the adhesion of the inserted glass tube to the flange faces and it is therefore the normal practice in this case for the lining to extend no further than the inner face of the flange. It is for that reason that many of the breakdowns in such tube systems caused by corrosion have, up to the present time, occurred at the flange joints.
The type of glass-lined metal tube most commonly used at the present time is produced by the glass tube insertion method illustrated in FIG. 1 below. In this case, a glass ring (3a), having a triangular cross-section, is inserted into an annular recess on the inner surface of the junction face (2) of the flange (1) so that, when it is heated and becomes soft, it will be fused with the end of the glass tube (5) which has been inserted into the metal tube (4). In this form of glass-lined metal tube, the glass material at the tube junctions will extend over only about one-half of the width of the flange-junction surface (2) so that, when the tubes are in service, permeation of corrosive fluid between those end faces (2) and gaskets will accelerate corrosion in the metallic parts of the periphery of the joints and will cause the glass ring (3a) to "float". This phenomenon is most marked in those locations where repeated heating and cooling makes it difficult to maintain the junction pressure at the flange faces and also in those locations where there are flanged junctions with, for example, glass-lined containment vessels in which there are curved sections in which the inner faces of the flanges have been molded or otherwise shaped. It is especially a problem in the case of joints with flanges of the type found in glass-lined containment vessels that, even if the curved inner faces of the flanges are precisely lined up with the above-mentioned glass rings (3a), and even when special gaskets are used, because of the virtual impossibility of ensuring a continuous surface of glass material at the joint area, this problem will not be entirely eliminated.
Furthermore, with this type of glass-lined metal tube, it will be necessary to grind off any projecting glass material from the flange faces (2) after cooling and the labor involved will increase in proportion to the size of the tube's diameter, with the added problem that there will be a risk of residual cracking of the glass material at those surfaces.
FIG. 2 illustrates an approach to the solution of the foregoing problems at the flange faces of this type of glass-lined metal tube. After applying a coating (3b) of either ceramic or glass material, by glazing and calcination, from the end face of the flange (1) to the tube interior, the inserted glass tube (5) is heated and softened so that its end will overlap the coating (3b) and, at the same time, adhere to the inner wall of the metal tube (4). With this approach, however, if the durability of the flange coating (3b) in a corrosive environment is to match the durability of the glass tube (5) that has been inserted in the metal sleeve, it will be necessary to repeat the glazing and calcination process of the flange coating (3b) at least four or five times in order to obtain the correct finish. This will lead to high costs. Furthermore, irregular breaking in the vicinity of curved portions on the inner sides of flange faces during cooling will expose the broken ends to corrosive fluids during service and there will be a risk of glass fragments being carried into the fluid.
The object of the embodied invention is to sustain the advantages of the glass tube insertion method and, at the same time, to overcome the problems listed above in connection with the flange faces.